A wireless device that is able to communicate with others using radio frequency (RF) signals is usually equipped with an RF transmitter and receiver. A conventional RF receiver may employ either a homodyne or a heterodyne receiver architecture. A homodyne receiver has its local oscillator (LO) frequency set to the same frequency as a carrier frequency in a received RF signal and makes a direct conversion from the RF carrier frequency to a baseband frequency for information recovery. Therefore, a homodyne receiver is sometimes referred to as direct conversion, or zero IF (meaning no intermediate frequency) receiver. The baseband frequency, or baseband, is in a range of frequencies occupied by the signal before modulation or after demodulation, and is typically substantially below the carrier frequency. Using the homodyne architecture, only one mixer stage is usually required to down-convert the received RF signal to baseband, resulting in lower power consumption and easier implementation of the receiver in an integrated circuit (IC) chip. However, because the LO frequency is tuned to the RF carrier frequency, self-reception may be an issue in a homodyne receiver.
In contrast, a heterodyne or superheterodyne receiver translates the RF carrier frequency in the received RF signal to one or more intermediate frequencies (IF) before demodulation. Modulation information is recovered from the last IF frequency. One or more LO signals are tuned to particular frequencies above or below the RF carrier frequency, and the RF and LO signals mix to produce the one or more IF frequencies. A heterodyne receiver is usually advantageous over a homodyne receiver because of its better selectivity and better immunity from interfering signals.
A heterodyne receiver, however, can be much more costly than a homodyne receiver. The received RF signal often includes unwanted signals that cause spurious responses at the IF frequency in addition to the desired signal. One spurious response is known to occur at a so-called image frequency. An RF filter known as a preselector filter is required to filter out the image signal unless an image-reject mixer is used. Moreover, additional crystal-stabilized oscillators are required for the heterodyne receiver. In addition to the added cost, these additional items also require extra receiver housing space and consume more power.
Traditionally, an RF receiver is designed with either the homodyne or the heterodyne architecture. There are situations, however, in which it is desirable to have an RF receiver that functions as a homodyne receiver for some operations and as a heterodyne receiver for some other operations. Such an RF receiver may be used, for example, in an RFID reader to facilitate a listen-before-talk (LBT) function required by the proposed European Telecommunications Standard Institute (ETSI) Standard, EN302 208-1.
RFID technologies are widely used for automatic identification. A basic RFID system includes an RFID tag or transponder carrying identification data and an RFID interrogator or reader, such as RFID reader, that reads and/or writes the identification data. An RFID tag typically includes a microchip for data storage and processing, and a coupling element, such as an antenna coil, for communication. Tags may be classified as active or passive. Active tags have built-in power sources while passive tags are powered by radio waves received from the reader and thus cannot initiate any communications. An RFID reader operates by writing data into the tags or interrogating tags for their data through a radio-frequency (RF) interface. An RFID reader for interrogating passive tags is typically designed to receive from the tags a backscattered portion of a signal transmitted from the reader and to extract information therefrom.
Conventionally, a homodyne receiver is the natural choice for an RFID reader during RFID operations because the received signal is merely a reflection of the transmitted signal and is thus at the same or slightly different frequency as that of the transmitted signal, with the slight difference being caused by, for example, a small Doppler shift. An RFID reader with the LBT function, however, is required to monitor in accordance with a defined listen time and immediately prior to each transmission for the presence of other signals within its intended sub-band of transmission, and can thus benefit from the superior performance, flexibility, and insensitivity to second-order distortion that are usually associated with a heterodyne receiver. But, it can be prohibitively expensive to incorporate two separate receivers in a low-cost RFID reader.